Summary of work: Cockayne syndrome (CS) belongs to the category of premature aging diseases, where the individuals appear much older than their chronological age. Cells from CS patients are sensitive to UV light, exhibit a delay in recovery of DNA and RNA synthesis following irradiation, and are defective in preferential repair and strand-specific repair of active genes (TCR). Complementation studies demonstrate at least two genes involved in CS, designated CSA and CSB. CSB protein, by sequence comparison, belongs to the SNF2 family of proteins, which have roles in transcriptional regulation, chromosome stability and DNA repair. The cellular and molecular phenotypes of CS include a significantly increased sensitivity to a number of DNA-damaging agents including UV irradiation. Transfection of the CSB gene into hamster cells with the CS-B phenotype completely restores TCR and UV resistance to normal levels, demonstrating that the defect in TCR in CS-B is due to mutation in that gene. The complex clinical phenotype of CS, however, suggests that DNA repair may not be the only defect. We have reported a defect in basal transcription in CS both in vivo and in vitro. This transcription defect is seen in CS-B lymphoblastoid cells and fibroblasts without any exposure to stress such as UV light. We have used an in vitro assay to measure the incision event of the DNA repair process. During the first step of base excision repair (BER) there is an incision in DNA 5' to the lesion. The incision can be quantified in cell extracts by using oligonucleotide duplexes that contain a single 8-oxoG lesion at a defined site. In CS-B deficient cell lines we observe a decrease in 8-oxoG incision that can be complemented by transfection of a plasmid containing the intact CSB gene. This suggests a direct role for CSB in the recognition of oxidative DNA damage and its repair. Further, we have analyzed the formation of another important oxidative DNA base lesion, 8-hydroxyadenine. The repair of this adduct is also deficient in CSB. We can conclude that the CSB protein is involved in the general genome base excision repair process, and that different domains of the helicase region play different roles in this process. We find that the repair defect that we observe directly correlates with cellular sensitivity to X-ray and that oxidative lesions accumulate in CSB cells after exposure. This could explain the high prevalence of neurological defects seen in CSB patients. The repair defect in CSB cells is not limited to the nuclear DNA, but also observed in mitochondrial DNA. Thus, remaining lesions in mitochondrial DNA in patients might contribute to the aging and neurodegenerative phenotypes. The role of CSB in the base excision repair pathway is further supported by our recent results concerning CSB protein interactions. In searching for proteins that bind and functionally interact with CSB we have shown that a major protein binder is PARP-1, a protein involved in the initial phases of DNA single strand break repair. CSB is a substrate for PARP-1 ribosylation and it is likely that these two proteins function together in the process of base excision.